verify my understanding of the code

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1337444 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/compress/compressors/bzip2/BZip2CompressorOutputStream.java

@@ -1321,7 +1321,10 @@ static final class Data extends Object {
         // ============
 
         /**
-         * Index in fmap[] of original string after sorting.
+         * Index of original line in Burrows-Wheeler table.
+         *
+         * <p>This is the index in fmap that points to the last byte
+         * of the original data.</p>
          */
         int origPtr;
 

src/test/java/org/apache/commons/compress/compressors/bzip2/BlockSortTest.java

@@ -0,0 +1,81 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.commons.compress.compressors.bzip2;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+
+public class BlockSortTest {
+
+    private static final byte[] FIXTURE = { 0, 1, (byte) 252, (byte) 253, (byte) 255,
+                                            (byte) 254, 3, 2, (byte) 128 };
+
+    /*
+      Burrows-Wheeler transform of fixture the manual way:
+
+      * build the matrix
+
+      0, 1, 252, 253, 255, 254, 3, 2, 128
+      1, 252, 253, 255, 254, 3, 2, 128, 0
+      252, 253, 255, 254, 3, 2, 128, 0, 1
+      253, 255, 254, 3, 2, 128, 0, 1, 252
+      255, 254, 3, 2, 128, 0, 1, 252, 253
+      254, 3, 2, 128, 0, 1, 252, 253, 255
+      3, 2, 128, 0, 1, 252, 253, 255, 254
+      2, 128, 0, 1, 252, 253, 255, 254, 3
+      128, 0, 1, 252, 253, 255, 254, 3, 2
+
+      * sort it
+
+      0, 1, 252, 253, 255, 254, 3, 2, 128
+      1, 252, 253, 255, 254, 3, 2, 128, 0
+      2, 128, 0, 1, 252, 253, 255, 254, 3
+      3, 2, 128, 0, 1, 252, 253, 255, 254
+      128, 0, 1, 252, 253, 255, 254, 3, 2
+      252, 253, 255, 254, 3, 2, 128, 0, 1
+      253, 255, 254, 3, 2, 128, 0, 1, 252
+      254, 3, 2, 128, 0, 1, 252, 253, 255
+      255, 254, 3, 2, 128, 0, 1, 252, 253
+
+      * grab last column
+
+      128, 0, 3, 254, 2, 1, 252, 255, 253
+
+        and the original line has been 0
+    */
+
+    private static final byte[] FIXTURE_BWT = { (byte) 128, 0, 3, (byte) 254, 2, 1, 
+                                                (byte) 252, (byte) 255, (byte) 253 };
+
+    @Test
+    public void testSortFixture() {
+        BZip2CompressorOutputStream.Data data = new BZip2CompressorOutputStream.Data(1);
+        System.arraycopy(FIXTURE, 0, data.block, 1, FIXTURE.length);
+        BlockSort s = new BlockSort(data);
+        assertFalse(s.blockSort(data, FIXTURE.length - 1));
+        assertEquals(FIXTURE[FIXTURE.length - 1], data.block[0]);
+        for (int i = 0; i < FIXTURE.length; i++) {
+            assertEquals(FIXTURE_BWT[i], data.block[data.fmap[i]]);
+        }
+        assertEquals(0, data.origPtr);
+    }
+}